The use of colorless recording systems dates back to the 1880's when Thomas Edison proposed the use of leuco Malachite Green on electrolytic telegraphic recording papers. This proposal was, however, slow to develop and it was not until 1938-40 that Groak et al. proposed the use of iron salts plus gelatin to form a carbonless carbon paper for use in recording. This Groak system involved separate coatings on separate sheets, or two insulated coatings on the same sheet. Unfortunately, Groak's paper was too hygroscopic (humidity sensitive) and too printing insensitive for practical use.
In the next important development, B.K. Green and coworkers proposed the use of Malachite Green Lactone as suggested by E. Weitz's publication on color reactions on clay surfaces and the use of the analogous compound, Crystal Violet Lactone with an acid-clay coated receiving sheet to form a colored print. Both of these reactions employed an acid-base reaction in which contact with acid transformed the colorless oxy base to a colored cation. As a first step, Green et al. emulsified the colorless base of the dye in a non-polar oil such as chlorinated diphenyl in a hydrophyllic colloid such as a gelatin solution in water. The encapsulated oil was then coated onto the surface of a top sheet in contact with an acid-clay coated bottom sheet. Pressure on the top sheet, as by striking with a pen or typewriter key, forced an oil droplet containing the colorless base onto the clay-coated receiving sheet to form colored indicia by the formation of the colored salt at those places where the colorless base was pressed against the acid-clay surface. Recent modifications of the Green et al. system use a polymeric acid on the receiving sheet to form a colored salt which is not discharged by contact with water. The chief drawback of this system is that the generation of a local excess of heat under writing pressure degrades polymeric acid to allergy-causing decomposition products.
Almost all of the prior art carbonless copy paper colorants have been acid-base color formers; that is, they are weak dye bases which react with, preferably, colorless acids to give colored dye salts.
An improved carbonless copy paper was proposed by Davis in 1965 (see Davis, U.S. Pat. Nos. Re. 30,797 and Re. 30,803; see also Heinisch Zollingen, Color Chemistry, 2nd Edition, Weiheim, N.Y., 1991, pages 393-6; and Rave, Ullman's Encyclopaediader Technischen Chemie, 4th Edition, Vol. 23, page 381, Verlag Chemie Weinheim). Unlike the previous systems, the Davis system utilized the principle of a colorless, associated dye salt which became colored on dissociation, conveniently by contact with the electrical ionizing field present on an unfired clay surface. This electrical ionizing field caused the dissociation of selected salts of Michler's Hydrol, particularly an organic sulfinic acid salt, to yield blue-colored indicia which were stable to both light and humidity. Approximately $1,000,000,000 worth of forms were sold employing the Davis invention up to the time when the system was superceded by a system which employed a mixture of three colorless dyes (leuco bases) whose colored ions collectively formed black indicia when contacted with a polymeric acid-coated sheet. There is, however, a major drawback to this three dye system; to wit, the three dyes fade under light at different rates, yielding, in most cases, magenta-colored indicia after some period of time.
The Davis colorless dye salt dissociation system has, to date, not been used with a mixture of colorless dye bases to yield a black print, and could not be used in an acid-base system.
At the present time, as mentioned above, all colorless copy papers which yield a black print similar to a black carbon paper print use a mixture of 2 or 3 color formers. What has long been desired is a colorless copy system (or carbon-less carbon paper) which uses only a single color-former to yield indicia with an intense blue-black to black shade on a suitable receiving surface.
Presently, world-wide yearly sales of colorless copy papers approximates $6,000,000,000.
It is an object of this invention to provide carbonless copy papers which yield black or deep blue-black indicia when subjected to the pressure of a typewriter key or pen or pencil.
Another object of this invention is to provide a heat-sensitive recording system for use in FAX machines to form a light-stable black to blue-black copy of the original document.
It is a further object of this invention to provide novel color bases or associated salts which can be converted from colorless to black or deep blue-black colors by reaction with oxidizing agents or under ionizing conditions which are capable of dissociating such undissociated salts, to yield a deeply colored cation. Other objects of this invention will appear from the following specification.